Despite the biocidal actions to kill the bacteria after attachment to the substrate through several mechanisms, such as inhibition of the synthesis of the cell wall, the nucleic acid, the protein and finally the distortion of the cellular metabolism (Murray et al. Medical Microbiology, 4th ed.; Mosby: St Louis, 2002), there are other more natural microbial prevention strategies that can be applied onto the polymer surfaces. While there might be others, one very good example of demonstration is the inherent antimicrobial property of chitosan, a natural polysaccharide derived from the shrimp (Chung et al. Acta Pharmacol. Sin. 2004, 25, 932). The most common antimicrobial products adopt bacteria killing as the strategy through biocidal actions and therefore the bacterial prevention effectiveness heavily depends on the release and replenishment of the leachable biocides that are required to migrate to the surfaces from the inner matrix. The biocides must dissipate off the surface for killing a microbe, leaving behind less antimicrobial effectiveness for future microbial encounters. This means that the microbial protection is perishable and has a fixed shelf life. Furthermore, over extended time, the adsorbed microbes on the substrate will gradually adapt to the surfaces, if the biocidal action is not immediate or if biocides are underdosed at levels below the minimum inhibitory (MIC) or minimum bactericidal concentrations (MBC). Each bacterial isolate has its specific MIC or MBC. This aids to select biocide-resistant microbes.
WO2005021626 discloses a built-in antimicrobial formulation for acrylic polymers by mixing with several organic antimicrobial additives in articles.
US20140008324 discloses methods for processing plastic substrates, comprising at least one of injection molding, thermoforming, or extruding, having inorganic antimicrobial microparticles within and then using plasma etching which results in the removal of a portion of the substrate surface and thereby exposing material within the substrate.
Photocatalysts, such as ZnO and TiO2, exhibit disinfecting effects against gram-positive and gram-negative bacteria and they work only under UV irradiation (Dhanalakshmi et al. Mater. Express 2013, 3, 291).
Others employ electrostatic method by the use of cationic polymers, such as chitosan, a natural polysaccharide. The bactericidal action of chitosan targets at the cell membrane of the bacteria which is negatively charged. The bacteria preferentially adsorb to the polymer surface and the strong adhesion leads to a gradual increase in their cell permeability and eventually intracellular dissolution due to the distortion of the charge distribution of the membrane (Chang et al. J. Agric. Food Chem. 2012, 60, 1837). However, this approach is not universal to target at a broad spectrum of bacteria bearing different cell membrane charges; some bacteria can be positively charged.
On the other hand, the lower-molecular weight polymers of chitosan were found to be able to diffuse and permeate through the porous membrane into the cells and form stable complexes with DNA. This subsequently prevents the DNA transcription activities, thus leading to inhibition of the proliferation and even the death of bacteria (Kenawy et al. Biomacromolecules 2007, 8, 1359).
Another biocidal approach requires complex nanoscale features and pattern topographies, such as nanopillars, which can be found on the insect wings (Pogodin et al. Biophys. J. 2013, 104, 835).
DE196060113 discloses a biocide-free coating free from biocides based on organofunctional silanes and fluoroorganosilanes and/or their hydrolysates and/or condensation products but the invention is limited to coatings which function properly with metal substrates, such as aluminum foil.
All these cater for the development of the new approach via a bacteria-repellent layer to be permanently and stably formed on the surface of a commodity plastic article and to prevent adhesion and accumulation of bacteria. This overcomes all the drawbacks that come with the conventional surface coating and/or combined with biocidal approaches.
One feasible antifouling structure is based on electrostatics via charge-bearing polymers, polyelectrolytes, polysaccharides and polypeptides containing amino, quaternized, carboxylated, sulfonated, phosphate, boronate entities and other metal oxides, complexes and their derivatives, of which the zeta potentials can be finely tuned by pH, counterion and charge valence.
U.S. Pat. No. 8,545,862 discloses an anionic/cationic polyelectrolyte complex, for example, a composition consisting essentially of a derivative or copolymer of poly(acrylic acid) or polystyrene sulfonate, to impart antimicrobial properties to an article.
WO2012065610 discloses a long-lasting antimicrobial coating for fabrics comprising a polymeric quaternary ammonium (quat).
EP2627202 discloses an antimicrobial peptide comprising Brad or an active variant thereof as a food preservative to prevent or inhibit spoilage of a foodstuff by a microorganism.
Just this approach, however, is not versatile enough. It is because some bacteria, plasma proteins and red blood cells carry negative charges on their cell membranes and they are expected to show very similar electrostatic repellent behavior against a substrate surface of the same charge. While other species of bacteria can be positively charged, such as Stenotrophomonas maltophilia, this approach is therefore not universal to target at a broad spectrum of bacteria as differentiated by the cell membrane charges.
The second feasible structure of antifouling groups is derived from neutral polymers, such as poly(2-hydroxyethyl methacrylate) (polyHEMA), poly(ethylene glycol) (PEG) and Zwitterionic polymers and a heterogeneous polymer system of mixed charges comprising of cationic and anionic functionalities onto the plastic surface (Sin et al. Polym. J. 2014, 46, 436). PolyHEMA shows the repellent properties because of its strong hydrophilicity so that it can displace the deposition of bacteria by a tightly bound hydration layer. Hydrophilic surfaces are apparently helpful to avoid bacterial adhesion. PEG however uses the steric exclusion effect to resist the protein and platelet adsorption. Previous data also suggested that the adherence of the bacteria was determined by the composition and the chemical nature of the pre-adsorbed protein layers coupled with the surface hydrophilicity. Zwitterionic polymers are bioinspired from the Zwitterionic phospholipid structures of the cell membranes which are well-known to be bio-inert. As different from the hydrophilic polymers, the betaine-based Zwitterionic polymers can finely tune the electrostatic interactions with the nearby water molecules and control the non-specific protein adsorption. Siedenbiedel et al. reveal successful examples of applications of Zwitterionic polymers for prevention of bacterial adhesion to the surfaces after chemical modification (Siedenbiedel et al. Polymers. 2012, 4, 46).
In the third feasible approach, the antifouling property of the plastic material can be achieved by modifying the chemical group functionality which in turn changes the surface hydrophobility via end termination and/or grafting of a polymer chain with alkyl, hydroxyl, fluoroalkyl and/or aromatic entities (Nie et al. J. Mater. Chem. B 2014, 2, 4911), completely different adhesion and physicochemical behaviors of biomolecules onto the surfaces will be acquired.